Because of their light-weight properties and excellence in heat insulation, flexibility, floatability and moldability, thermoplastic resin foams have been widely used as various heat insulating materials such as for roof insulations, roof covers for vehicles or floor insulations, cushioning materials, floating materials, or profiles.
In obtaining such thermoplastic resin foams, prior manufacturing methods typically heat a sheet of expandable thermoplastic resin containing a heat-decomposable blowing agent to a temperature sufficient to decompose the blowing agent to thereby effect foaming thereof. The foaming of expandable thermoplastic resin sheet is effected by the action of pressure of gases released as the blowing agent contained therein decomposes. Accordingly, the expandable thermoplastic resin sheet, when subjected to foaming, generally expands substantially three-dimensionally in a uniform fashion. In the manufacture of thermoplastic resin foams, especially in the continuous manufacture of elongated thermoplastic resin foams, a measure is needed which reduces the production of wrinkles in the resin foams due to expansion thereof in width and length directions.
In the Japanese Patent Publication No. Sho 48-9955, for example, a method is disclosed which contemplates to reduce wrinkles in a thermoplastic resin foam that will be finally obtained. The method involves feeding a continuous sheet of expandable thermoplastic resin containing a blowing agent for subjecting to heat foaming to obtain a thermoplastic resin foam which is subsequently taken up. This take-up rate is increased relative to the feed rate of the expandable thermoplastic resin sheet, in such a proportion as to correspond to the lengthwise increment of the resin sheet that will be gained as it expands, while the thermoplastic resin foam is stretched in its width direction in such a proportion as to correspond to the widthwise increment of the resin sheet that will be gained as it expands.
However, this method requires complicated jigs and processes for widthwise stretching of the continuously produced thermoplastic resin foam which has been just heated and foamed. In addition, the requirement to stretch the thermoplastic resin foam before it is cooled leads to the reduction in quality of both lateral ends of a resulting thermoplastic resin foam. This necessitates the removal of those lateral ends from the resulting thermoplastic resin foam, which problematically drops the productivity thereof.
Since the above method employs a continuous sheet of expandable thermoplastic resin to form the thermoplastic resin foam, the thermoplastic resin foam thus formed is imparted thereto excellent thickness precision, weight precision and surface smoothness. However, due to its homogeneity in its thickness direction, the resulting thermoplastic resin foam has a problem of poor compressive strength.
Also, in the thermoplastic resin foam obtained in accordance with the above-described method, the planar expansion of the thermoplastic resin caused when subjected to foaming is counterbalanced by the lengthwise elongation and widthwise stretch of thermoplastic resin foam which accompany the forces that will remain therein as a thermal stress. Thus, the resulting thermoplastic resin foam, if subjected to change in temperature, is caused to change its size to a problematically large extent.
Furthermore, the attempt to obtain an irregularly-surfaced thermoplastic resin foam by utilizing the above-referred method results in the necessity of post-forming which provides irregularities to a previously fabricated, plate-form thermoplastic resin foam, and accordingly fails to manufacture the irregularly-surfaced thermoplastic resin foams at high productivity. Due to the homogeneity in its thickness direction, the irregularly-surfaced thermoplastic resin foam obtained also had a problem of insufficient compressive strength.
On the other hand, Japanese Patent Laying-Open No. Hei 7-16856 discloses a method which obtains a sheet-form thermoplastic resin foam by spreading pellets or rounded mass (hereinafter referred to as pellets or the like) of expandable thermoplastic resin containing a blowing agent over a conveying belt, and heating the pellets or the like of expandable thermoplastic resin to effect foaming and expansion for fusive integration thereof.
In accordance with this method, a thermoplastic resin sheet or another conveying belt is provided above the conveying belt to define therebetween a restricted space within which the dispersion of pellets or the like of expandable thermoplastic resin over the lower-conveying belt are allowed to be heat foamed, so that a thermoplastic resin foam having a desired thickness can be obtained. Concurrently, the spaces among the pellets or the like of expandable resin, defined in a planar direction of the sheet, are filled as they expand, thereby obtaining a sheet-form thermoplastic resin foam.
Again, in this method, the expandable thermoplastic resin, when subjected to foaming, expands three-dimensionally. However, since the pellets or the like of expandable thermoplastic resin are arranged two-dimensionally in an intermittent manner to provide spaces thereamong, the subsequent, two-dimensional expansion of the pellets or the like of expandable thermoplastic resin fills those spaces. That is, the foam is obtained through the pseudo-one-dimensional expansion of expandable thermoplastic resin in its thickness direction. This eliminates the necessities of widthwise and lengthwise stretch or elongation.
However, if this method is to effect foaming in the form of a pseudo-one dimensional expansion, the spaces must be predetermined which accommodate the increments in area of the pellets or the like of expandable thermoplastic resin as they expand. In order to predetermine the size of spaces, the dispersion of pellets or the like of expandable thermoplastic resin must be controlled with extremely high precision. A spreader thus becomes necessary which can disperse the pellets or the like of expandable thermoplastic resin with precision.
Also, this method contemplates to obtain a sheet-form thermoplastic resin foam by heat fusing the intermittently arranged pellets or the like of expandable thermoplastic resin to unity while subjecting them to foaming. It however presents a problematic possibility that those pellets or the like of expandable thermoplastic resin may not completely be fused to unity. Accordingly, high productivity can not be expected from this method. In addition, if desired to obtain a thermoplastic resin foam having an increased thickness, the size of pellets or the like of expandable thermoplastic resin must be enlarged accordingly. In such an instance, the necessity of uniformly heating the large-sized pellets prolongs the foaming period to thereby reduce productivity.
It is within the knowledge of the present inventors that the spreading of pellets or the like of expandable thermoplastic resin must be controlled with a still higher precision, if particularly desired to obtain an irregularly-surfaced thermoplastic resin foam. That is, where the spaces are determined as excessively larger than for corresponding to the increments in area of the pellets or the like of expandable thermoplastic resin as they expand, there remains a possibility that the pellets or the like of expandable thermoplastic resin, if heat foamed, may not completely be fused to unity. On the other hand, if the determined spaces are smaller than for corresponding to the increments in area of the pellets as they expand, a thermoplastic resin foam will be obtained which is not irregularly-surfaced but in the form of a flat plate.
Accordingly, the aforementioned method requires a spreader for dispersing the pellets or the like of expandable thermoplastic resin in such a highly precise fashion as stated above, and has been extremely difficult to obtain a thermoplastic resin foam having a predetermined irregular configuration.
Also in accordance with the above manufacturing method, an individual pellet or,the like of expandable thermoplastic resin, when foamed, produces a skin layer of a low expansion ratio on its surface, resulting in the formation of a thermoplastic resin foam in which highly-expanded portions of thermoplastic resin having peripheries completely covered with respective slightly-expanded thin films of thermoplastic resin are heat fused to each other through the slightly-expanded thin films. The resulting thermoplastic resin foam has a high degree of compressive strength, but its qualities such as thickness precision, weight precision and surface smoothness, as well as the variation in compressive strength, depend largely upon how the pellets or the like of expandable thermoplastic resin are dispersed.
Also in accordance with the above manufacturing method, a pressure developed during foaming acts to fuse bond the neighboring highly-expanded portions through the respective slightly-expanded thin films to thereby assure sufficiently strong fusion bonds. However, the absence of foamed layers continuous in a planar direction of the thermoplastic resin foam obtained, as well as small fusion bond areas, reduces the strength thereof against a flexural load, if applied, to the degree that can not be said to be sufficient.